Herpesviruses are human pathogens that cause life-long persistent infections and result in clinical manifestations that range from a mild cold sore to cancer. Herpes simplex virus (HSV) is among the most frequently encountered virus by the general population. Infection with HSV-1 often results in orolabial disease but it can also cause ocular disease (keratitis) that can lead to blindness or encephalitis, which can be fatal. Type-2 infections are characterized by genital ulcerative disease and infections of the newborn. Thus, infections due to HSV are a major public health concern and understanding the biology of this virus is important in the development of efficacious treatments of these infections. The HSV-1 virion is comprised of four structural elements: a large double-stranded DNA molecule in the central core; an icosahedral capsid, which encloses the genome; a layer that immediately surrounds the capsid termed the tegument; and an outer membrane or envelope, which encloses the whole structure and in which are embedded the viral glycoproteins The tegument is one of the most complex and diverse structures of the virion, both in terms of protein composition and the functions encoded by the constituents of this structure. Of this large and diverse group of virion proteins, not all are classified as being essential for replication in a laboratory cell culture system. What are the functions of all the ?non-essential? proteins for virion maturation and egress? These ?accessory? gene products likely play important roles in the infected host but an in-depth discovery of their functions has not been accomplished in a comprehensive manner. Using a genome assembly line coupled with synthetic genomics methods, we can elucidate the functions of these proteins by creating ?synthetic lethality? in a mutant virus. Our working hypothesis posits that by phenotypic investigation of synthetic lethals, we can then begin to formulate pathways in which these ?redundant? virion proteins participate, as well as, their spatial/temporal roles during HSV-1 virion morphogenesis. The outcome of the proposed studies is expected to further our understanding of the roles of these multitude virion proteins and the importance of their interactions for infectious particle formation. The Specific Aims proposed to achieve these goals are: Specific Aim 1: Genetic interaction networks of the conserved tegument genes: Pivotal role of pUL21. Specific Aim 2: Elucidate the phenotypes of ?synthetic lethals? that are centered around the abundant tegument protein, VP22 to determine pathways that lead to virus maturation and envelopment.